To protect digital content such as video and music (hereinafter, “content” refers to digital content) against unauthorized copy, various organizations have proposed their copyright protection standards. Such standards usually include robustness rules which are specifications indicating criteria for implementation. The robustness rules demand implementation of a copyright protection technology that can prevent unauthorized tampering or analysis by a malicious user.
The copyright protection technology according to the robustness rules is usually implemented by hardware or using a tamper-resistant technique constructed by the software described in Non Patent Literatures 1 and 2. The tamper-resistant technique makes reading difficult by performing compiling in the state where an unnecessary program code is inserted, for example. As another copyright protection technology, a technique of switching an execution mode of a Central Processing Unit (CPU) as shown in Non Patent Literature 3 is used in recent years. The implementation of such copyright protection standards is usually performed in the field of Personal Computers (PC).